The power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and the IGBT (Insulated Gate Bipolar Transistor) as a discrete element used for switching power supply and the like are required to have high speed operating characteristics and low on-resistance properties. High speed operating characteristics are achieved by a planar structure having low capacitance characteristics.
For example, the MOSFET having a planar structure disclosed in Patent Document 1, as shown in FIG. 5, comprises a plurality of gate electrodes 2 formed in a stripe shape on an N type semiconductor board 1, P− type base layers 3 and N+ type source layers 4 formed in a self-aligned manner by double diffusion with respect to the gate electrodes 2, an interlayer insulating film 5 to cover the gate electrode 2, and a source electrode (not shown) formed of metal film and formed on the interlayer insulating film 5. A plurality of P− type base layers 3 and a plurality of N+ type source layers 4 are formed in a stripe shape along the gate electrodes 2. On the N+ type source layer 4, the interlayer insulating film 5 is formed with stripe shaped contact holes 7 along the entire length in a longitudinal direction of the source layer 4. The source electrode enters through the contact hole 7 to form an ohmic junction with the source layer 4.
With such structure, after the common connection of the plurality of gate electrodes 2 is made and the gate electrodes 2 are applied with a voltage larger than the predetermined threshold voltage, a surface portion of the base layer 3 is formed with an inversion layer and electricity is conducted between the semiconductor substrate 1 (drain) and the source layer 4, thus achieving transistor operation.    Patent Document 1: Japanese Unexamined Patent Publication No 8-321605